Spinal fixation assemblies may be used to align and/or fix a desired relationship between adjacent vertebrae. Such assemblies generally include a connecting rod that is attached to a plurality of vertebral bodies by an anchoring device which may include hooks, bolts, wires, or screws. The connecting rod can have a predetermined shape or contour that has been designed according to the properties of the target implantation site, and once installed, the rod secures the plurality of vertebral bodies in a specific spatial relationship to allow bone fusion to occur.
Spinal rods can be anchored to various anatomic aspects of the vertebral bodies, but because of the size and contour variances seen in the spine, a variety of anchoring devices have been developed and are commercially available. Pedicle screw assemblies are an example of an anchoring device and typically include a bone screw that has a threaded shank and a head portion that is connected in some manner to a receiving element. The receiving element may include a portion that is usually in the form of a U-shaped slot that accepts a connecting rod. A threaded set-screw or cap may be used to lock the rod into the receiving element that is coupled to the bone screw.
In order to clinically address the vertebral size and anatomic differences seen within the various portions of the spinal column, pedicle screw systems that are used currently frequently incorporate designs that require complex assembly steps which increase the cost of manufacturing and difficulty with intraoperative assembly. In addition, current pedicle screw system designs typically require a high tightening torque to achieve an adequate lock on the bone screw implants. As a result of the high tightening torque force requirements, larger implant profiles may be necessary to withstand the resultant loads and cause disruption or fracture at the screw-bone interface. Further, a continuing unsolved problem exists with previous pedicle screw designs with the splaying or separation of receiving element or housing when a top screw or threaded member is threaded into or engaged with the housing to apply an adequate load to the rod that is being held within the housing.
Accordingly, there remains a need for a pedicle screw assembly that is bottom loading to allow for adequate fixation of the rod to the receiving element while not disrupting the bone-bone screw interface during implantation or splaying of the receiving element when set screw securement occurs.